You can always buy and install Windows from scratch, if that is what you are asking.
Chances are pretty good you can get your old copy reactivated. Only concern would be the drivers and getting Windows to boot properly.
If you list the motherboard you have, and what you plan to get, along with some basic system configuration details might be able to get a clearer answer.
I have transferred a lot of OS to new hardware platforms. Only time I had real difficulties that ended up in a re-install was with a computer that had DOS, XP, and Windows 7 spread across two drives and 3 partitions. Couldn't remove the old boot drive.